In general, an OFDM synchronization method in a broadcasting/communication system is divided into a frequency synchronization method and a time synchronization method. In the frequency synchronization method, a carrier frequency offset between the transmitter and the receiver and a frequency offset caused by the Doppler effect generated due to movement of the receiver are estimated and compensated. In the time synchronization method, a start position of an OFDM symbol is estimated and compensated.
In an OFDM system, a frequency offset causes inter-channel interference (ICI), which breaks the orthogonality between multi-carriers. Therefore, performance of a receiver is lowered.
In the time synchronization method, when a start position of a symbol is not correctly estimated, inter-symbol interference (ISI) occurs, and therefore, performance of a receiver is also lowered. Conventionally, such a problem was overcome using a cyclic prefix.
That is, a cyclic prefix is inserted into a symbol so that a serious error is not generated even though time synchronization is not correct.
However, performance of the receiver is very sensitive to frequency offset and considerably influenced by the accuracy of frequency synchronization.
Hereinafter, such conventional problems will be described in detail with reference to a terrestrial digital multimedia broadcasting (T-DMB) system using an OFDM synchronization method.
FIG. 1 shows the structure of a transmission frame in a conventional prior art T-DMB system. As shown in FIG. 1, the transmission frame in the T-DMB system comprises a synchronization channel 110, a fast information channel (FIC) 120 and a main service channel (MSC).
The synchronization channel 110 is a channel for transmission frame synchronization, and comprises a null symbol 111 which has no signal and a phase reference symbol 112 which becomes a reference for decoding DQPSK signals. The null symbol 111 is a period in which there is no signal, and a receiver obtains approximate time synchronization using an energy ratio of the null symbol 111 and the PRS 112. The PRS 112 contains information known by both receiver and transmitter. Accordingly, the receiver can obtain precise time synchronization and exact fractional frequency synchronization using the information.
The T-DMB transmission frame comprises a total of 76 (PRS+FIC+MSC) OFDM symbols 140 except for the null symbol 111. A cyclic prefix (CP) 141 is inserted into each of the OFDM symbols 140.
The CP 141 is a kind of guard time for extending an OFDM symbol period, and the continuity of a signal is maintained by copying an end of the OFDM symbol 140 and inserting the copied end into a head of the OFDM symbol 140. Accordingly, although a receiver does not obtain precise time synchronization, an error is not generated in a data decoding process due to the CP.
The receiver estimates a fractional frequency offset using properties of the CP 141.
The frequency synchronization method includes a closed-loop synchronization method and an open-loop synchronization method. The closed-loop synchronization method is a method in which a frequency offset is compensated by adjusting the frequency of a local oscillator to the opposite direction using a frequency offset that is estimated in a signal processing stage and feedback to the local oscillator.
In the closed-loop synchronization method, the cause of a frequency offset is removed, but estimation of a frequency offset and compensation time through a feedback loop are delayed as compared with those in the open-loop synchronization method. Therefore, changes in frequency offset are not rapidly managed. Further, performance is different depending on characteristics of the local oscillator.
On the contrary, the open-loop synchronization method is a method in which a frequency offset is estimated and immediately compensated in a signal processing stage.
In the open-loop synchronization method, a load for performing signal process in a digital stage is high. However, performance is not sensitive to characteristics of an oscillator and constant performance is maintained like in the closed-loop synchronization method. Further, since compensation for a data having an estimated frequency offset is immediately possible, there is no time delay for compensating for a frequency offset.
FIG. 2 shows the structure of a T-DMB receiver to which a conventional open-loop frequency synchronization method is applied.
As shown in FIG. 2, the T-DMB receiver comprises an antenna 201, a low noise amplifier (LNA) 202, a mixer 203, a local oscillator 204, a band pass filter 205, an analog-to-digital converter (ADC) 206, a baseband digital signal processing unit 207 and an A/V codec 208.
The LNA 202 amplifies signals inputted to the antenna 201, and the mixer 203 transfers the amplified signals into a baseband signal. At this time, the mixer 203 uses a signal generated from the local oscillator 204. When the carrier frequency of the signal is different from that of a transmitter, a frequency offset is generated.
The band pass filter 205 removes an unnecessary signal in the amplified signals. The ADC 206 converts the amplified signals into digital signals and outputs the converted signals to the digital signal processing unit 207.
The digital signal processing unit 207 comprises a time offset estimator 271, a fractional frequency offset estimator 272, an integer frequency offset estimator 273, a time/frequency offset compensator 274, an OFDM demodulator 275 and a channel decoder 276.
The time offset estimator 271 estimates an OFDM symbol start position and a frame start position. The fractional frequency offset estimator 272 estimates a frequency offset that becomes fraction times of the interval of a sub-carrier. The integer frequency offset estimator 273 estimates a frequency offset that becomes integer times of the interval of a sub-carrier. The time/frequency offset compensator 274 adjusts the frame and OFDM symbol start positions and compensates for a frequency offset using the estimated time and frequency offset information.
The OFDM demodulator 275 demodulates a digital signal using FFT, and the channel decoder corrects errors of the digital signal. The A/V codec 208 restores the digital signal to video and audio signals.
Here, an algorithm used in frequency offset estimation, i.e., an algorithm used in integer frequency offset estimation will be described. A position of a subcarrier on a frequency axis that gives a maximum correlation value between a knwn PRS and a received PRS is detected while moving the received PRS a signal on a frequency axis as the interval of the sub-carrier. Here, the spacing difference between the position of the known PRS and that of the received PRS that gives a maximum value becomes an integer frequency offset.
At this time, since performance of the algorithm can be deteriorated due to the influence of a time offset, an algorithm based on a partial correlation value may be used to remove the influence of the time offset.
In the T-DMB system, the signal known by the receiver is only the PRS. Therefore, the integer frequency offset can be estimated using PRS in the frame once.
In the fractional frequency offset estimation, properties of a CP are used on a time axis, unlike in the integer frequency offset estimation. The CP is formed by copying an end of an OFDM symbol and inserting the copied end into a head of the OFDM symbol. The fractional frequency offset using a CP is estimated using a phase difference between data in a received signal, which is caused by a frequency offset.
An expression for the fractional frequency offset estimation is as follows;
                              F          ⁢                                          ⁢          F          ⁢                                          ⁢          O                =                              1                          2              ⁢                                                          ⁢              π                                ⁢                                    tan                              -                1                                      ⁡                          [                                                ∑                                      n                    =                    0                                                                              N                      g                                        -                    1                                                  ⁢                                                                            z                      r                                        ⁡                                          (                                                                        -                                                      N                            g                                                                          +                        n                                            )                                                        ·                                                            z                      r                      *                                        ⁡                                          (                                                                        N                          e                                                -                                                  N                          g                                                +                        n                                            )                                                                                  ]                                                          [                  Equation          ⁢                                          ⁢          1                ]            
Here, the range of a fractional frequency offset is limited by the aforementioned characteristics of the algorithm. That is, it is possible to perform estimation up to −0.5*Lss to +0.5*Lss (Lss: interval of a sub-carrier) due to the property of tan−1.
FIG. 3 shows various frequency offset values to be estimated by a fractional frequency offset estimation algorithm. Since FF01 and FF02 are within a range of the fractional frequency offset estimation, they can be estimated. On the other hand, since FF03 and FF04 are out of the range, they are estimated as FF05 and FF06, respectively.
When FF01 is changed into FF03 due to the aforementioned shift of the local oscillator, FF01 is out of the range of the fractional frequency offset estimation and estimated as a wrong value. Therefore, frequency synchronization is not correct, and data is not properly decoded by the receiver.